1. Field of the Invention
The present invention relates to a device for inserting characters in a video signal, the characters being able to be displayed transparently against a background consisting of the normal television image. The invention is particularly advantageous when the analog video signal corresponding to the background image is coded according to a standard dictating that the chrominance information be frequency modulated.
The invention relates to the field of television and its peripheral applications.
2. Description of the Related Art
Video appliances (video recorders, video laser disk players, decoders) increasingly rely on the display of text on the screen to facilitate exchange of information with the user. Such text may include programming menus, status messages, teletext, to mention but a few examples. For this purpose, the appliances comprise a character generator, the role of which is to insert the information corresponding to the text to be displayed into the video signal.
The text generation device as a whole then supplies either a RGB signal, or a signal coded according to a particular standard (for example PAL, NTSC, SECAM). Text insertion, for example into a composite video base band signal (called CVBS signal from now on) is done in a way known to the person skilled in the art, by multiplexing the CVBS signal with luminance plateaux corresponding to the characters to be displayed. Thus, at defined moments, the video information of the CVBS signal (luminance and chrominance) is replaced by different luminance information.
The introduction of the plateaux erases the colour information (chrominance) initially present. According to the PAL and NTSC standards, chrominance is amplitude modulated. During luminance plateaux due to the text, the amplitude of the chrominance signal is zero; the text on the screen is simply a more or less dark grey. In SECAM, the chrominance is frequency modulated. However, the luminance plateaux, generally high, introduce frequency variations into the CVBS signal. In consequence, numerous chrominance defects are seen to appear on the screen, defects which give rise to considerable problems in reading the text and/or in terms of visual comfort.
One known solution used to reduce the chrominance defects consists of sampling the chrominance information in the input CVBS signal, before the insertion of text, by virtue of a band pass filter targeted on the central frequency of the chrominance. This information is then reinjected into the video signal containing the inserted text. This practice is used in so-called "MIXED MODE" operating mode, one part of the screen including text, the other part the image. The text then takes the colour of the image. The luminance level of the text is chosen to be high in order to exhibit good visibility. This solution exhibits drawbacks. The amplitude of the chrominance information varies, in effect, both when text is present and when a part of the original CVBS signal is present, which can cause a degradation in the signal. Moreover, it is preferable, in multi-standard systems, to implement the bandpass filter only for a standard of the SECAM type, for which the chrominance is frequency modulated. That being so, a switch is provided at the pickoff point of the low-pass filter in order to put the latter out of service at the desired time. It is necessary, moreover, to provide logic for control of the switch, which should thus open when it is desired to display a text in the absence of any CVBS signal, or when the video signal is derived solely by the text generator, in order to prevent the CVBS signal disturbing the text. This latter mode is called "FULL-PAGE MODE". Moreover, the low-pass filter includes a capacitor and a choke, elements which are difficult to integrate.
The cost of this solution is therefore not inconsiderable.
In devices employing the usual character generators, the inserted text completely blanks the corresponding parts of the image. This can be troublesome, especially in the event that a menu for setting the image by remote control blanks out a large part of this image, or when it is desired to keep a text displayed while wishing to follow the normal program.